This Phase II, randomized, double-blind, two arm, outpatient, AIDS Clinical Trial Group (ACTG) study is designed to evaluate the safety and tolerance of ZDV monotherapy vs. d4T monotherapy administered to antriretroviral-naive symptomatic HIV-infected infants. Infants and children with vertically-acquired HIV infection often experience more rapid immunologic deterioration and disease progression than is observed typically among HIV-infected adults. The study will also compare the clinical responses between the two treatment groups as defined by disease progression; compare virologic and immunologic responses between the two treatment groups as measured by changes in viral burden, p24 antigen concentrations, and CD4 T-cell counts; and obtain population pharmacokinetic parameters for both study drugs.